Condenser is a large lens mounted below the stage, with
an iris and diaphragm. There may be 2 or more lenses. Its
function is to deliver the light beam to the objective at a
On objective quality, depends, the quality of the image.
These are usually made up of more than one lens. On each
objective is engraved the magnification power.
Numerical aperture (NA) of the objective is important, for
on this, depends, among other things, the amount of light
which the lens passes and the detail which it can make
visible, on which it is said to resolve.
They are used to avoid bending of light beam (with higher
magnification). The oil used should have the same optical
properties as glass, e.g. cedar wood oil. Liquid paraffin can
With increasing magnification certain optical aberrations
1. Spherical aberration—edge of the lens gives slightly
higher magnification than its center.
2. Chromatic aberration—blue light is magnified slightly
These aberrations can be avoided by using a series
of lenses made of special glass, carefully calculated and
1. Achromatic—are the usual average quality lenses and
are good enough for routine laboratory work.
2. Fluorite (Fi)—are highly corrected and expensive, have
a wider field and are good for searching blood films. FIG. 1.9: Microscope objectives
20 Concise Book of Medical Laboratory Technology: Methods and Interpretations The Mirror
It is placed below the condenser and iris, it can be turned in
any direction. It reflects the light beam from the source to
the iris and condenser. It usually has two mirrors mounted
back to back, one flat and the other concave. Flat mirror
is used in the presence of condenser and the concave
Use of direct sunlight is bad for the microscope and the
eye. It is best to use reflected sunlight of a dull white
background. It is not sufficient for oil immersion lens and
it is not available during evening or night.
A 60 watt frosted electric lamp placed 18" away from the
microscope is sufficient for most routine work. Many
microscopes are now provided with built-in sources of
illuminations. In the absence of electricity, a battery lamp
or an oil lamp can be utilized. The light from these artificial
sources is rather yellow but may be used. Best, however,
Special Applications of the Microscope
This is needed to visualize transparent microorganisms
suspended in a fluid. Ray of light travels in a wave form in
a straight line. Two such rays traveling together are said to
be in phase, and they produce a brighter illumination. If,
however, these rays are out of step with each other, they
are said to be out of phase. They interfere and produce less
bright illumination. Phase contrast microscopy makes use of
this property of rays to help or hinder each other and thereby
resulting increased contrast in the microscopic image.
The desired effect is brought about by placing an
annulus in the condenser and a phase plate in the
objective. A circle is engraved in the phase plate which
matches the ring of beam coming through the condenser
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